In the testing of lung capacity and the ability of a person to inhale or exhale it is known to use an inhalation valve to produce a change or differential in air pressure. One suitable inhalation valve is shown in U.S. Pat. No. 4,456,016. The differential in air pressure is pneumatically connected to a transducer to produce an electric voltage which is a function of the change or differential in air pressure. The change or differential in air pressure produced is not very great, and in the past this has required very sensitive transducers that have been quite expensive. Less expensive transducers are known, but are much less sensitive. In U.S. Pat. No. 4,602,171 there is disclosed a suitable electronic circuit for use in connection with an inexpensive transducer having a relatively low output voltage.
There are times when a patient is unable to breath for himself, and he is placed on a breathing apparatus providing forced inhalation. In such instance it is preferable to measure the exhalation, since this is done voluntarily by the patient. In such a situation the valving for the valve may be reversed, or a Venturi device may be used without a valve. The important thing is that a transducer is provided which converts air pressure differential into a low level electrical circuit.
It is known that the output of the transducer drifts with temperature and time. Both long term drift and short term drift are important, and both could produce erroneous results. In the co-pending application for U.S. Pat. Ser. No. 809,393 filed Dec. 16, 1985, now abandoned, by Alfred G. Brisson, Christopher Nowacki, and Thomas H. Burdick, assigned to the same assignee as the present application, namely Trutek Research, Inc., of Lake Zurich, Ill., there is disclosed a compensation circuit for calibrating to compensate for drift when the circuit is initially turned on, and then over a relatively long time constant period, on the order of 50 seconds, which is about ten times the length of the respiratory signal, and effects such compensation only when there is no signal in. The circuit in the aforesaid co-pending patent application is an analog circuit, and relies on the charging or discharging of a capacitor to a requisite reference voltage. This circuit generally works well, but is subject to the usual problems of analog circuits.